To use automatic updates you need to use Envato Market plugin and set Envato API Token in plugin.
For more information about its configuration, please read this article: “How To Automatically Update WordPress Themes with the Envato Market Plugin”.
When new update will be released, the ThemeForest will notify you via email, so you can download new package and install it manually. We don’t recommend this approach. The more robust way is to use automatic update.